Conductive Additive Electric Current Sintering

ABSTRACT

The present disclosure is directed to an electric current sinterable material containing a minority portion being significantly more electrically conductive than the primary material being sintered. This includes forming an inorganic body or sintered coating as well as an apparatus for and method of making use of such a variable composition powder. An electrical current is used to cause a combined energy and temperature profile sufficient for powder-powder sintering. This preferred method for powder-substrate bonding is referred to as flame-assisted flash sintering (FAFS).

CROSS REFERENCE TO RELATED CASES

This application claims the benefit of priority of U.S. Provisional Application Ser. No. 62/189,075, filed on Jul. 6, 2015. The entirety of that provisional application is hereby incorporated.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No. FA85650-14-C-2425, awarded by the United States Air Force. The Government has certain rights in this invention.

BACKGROUND

1. Field of the Disclosure

The present disclosure is directed to compositions of a mixture of materials that enhance processes where an electric current is passed through the material, resulting in enhanced sintering. Proof of functionality was performed on sintering coatings onto a surface or substrate. There are several ways of establishing current flow through materials, including the use of flame-assisted flash sintering (FAFS), which involves a flame with an electric field plasma. Many materials are non-conductive or very poor conductors of electric currents, and the current innovation comprises adding a small amount of a more conductive material to enable more uniform sintering, higher current in a similar applied field, or sintering at lower initial temperatures. Using starting materials that react during the sintering with a material with higher resistance, the sintering can be self-limiting and sintering will move onto areas that have not fully changed. The preferred method is capable of being used in an ambient atmospheric environment and can be used with no additional heat source other than the flame or plasma. A wider range of materials can be thus processed with the addition of conductivity modifiers, and substrates can be exposed too much lower temperatures allowing for a wider range of materials and with less residual stresses from thermal expansion. Such control has beneficial properties and uses including allowing for sintering of ceramics onto a wide range of substrates including low melting materials such as aluminum alloys.

2. Background of the Disclosure

The capability to sinter ceramics more rapidly and at lower temperatures than with conventional processes is highly desirable. Ceramic coatings on metallic substrates serve many purposes in many applications because ceramics can provide desirable wear, hardness, chemical, appearance, wetting, thermal, and electrical properties. Because ceramic materials generally have superior hardness, with better temperature and corrosion resistance, than metals, ceramics can extend the life of heat exchangers operating in extreme environments, for example. However, ceramics are typically brittle and usually have different thermal expansion properties, which can lead to a sintered coating cracking.

Ceramic coatings are also important for performance and longevity in thermal barrier coatings (TBC) for gas-turbine engines, among other applications. The hot gas streams in gas-turbine engines can reach temperatures well in excess of 1000° C. and a barrier coating is thus necessary to protect the underlying metal from corrosion and, for TBC, thermally insulating coatings are helpful.

Numerous other applications are known to benefit from ceramic coatings onto metals, including fuel cells, battery-electrode coatings, wire-insulation coatings, wear and abrasion surfaces, cookware, engines, exhaust shields, power plants of various types, biomedical implants, surfaces exposed to supersonic gas flows, and other aerospace applications.

Two common methods used to deposit ceramics onto metals are air plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD). In APS, ceramic powder is injected into an acetylene-oxygen flame nozzle that contains a plasma arc formed by a voltage and the high temperatures generated from the combustion process. As the powder feedstock is injected through this hot region (>2500° C.), the powder melts and some consolidates into large droplets that are then conveyed to the metal substrate where they splat-impact, cool, and resolidify.

The porosity and smoothness issue are improved when using EB-PVD, where an intense beam of electrons melts and vaporizes a solid ceramic target inside a vacuum chamber. As a melt is formed, vapor-phase material is generated within the low-pressure chamber and a uniform coating is deposited on a nearby substrate. Although this process deposits films that are generally superior to APS, the method is costly, because it is slower and requires expensive vacuum chambers, source targets, and power supplies for beam generation and steering. Moreover, in any vapor-phase deposition, a large percentage of the target material becomes wasted and deposited on the surrounding chamber walls and because the process is line-of-sight, the substrate must be manipulated in the vacuum chamber to coat all the surfaces. Thus, cost is a limiting issue with EB-PVD and it is only used for the most demanding applications. Plasma-enhanced chemical vapor deposition (PECVD) is a similar technique in that it is a low-pressure vapor deposition process, but suffers from some of the same cost issues as EB-PVD, except that it is not as limited in line-of-sight and the substrate is exposed to a high temperature deposition chamber. These vapor deposition processes do not provide for coating of large or assembled parts since they must be in a deposition chamber. There also is no sintering of material as is the case with all vapor deposition processes.

Various techniques exist that use electric fields to sinter ceramic materials. Such techniques are collectively referred to as “field-assisted sintering” (FAST), and include spark plasma sintering (SPS), pulsed electric current sintering (PECS), and flash sintering. In all of these methods, an electric field is applied across a green body material and resistive heating caused by current flow consolidates the powder material. Traditional SPS applies uniaxial pressure to a ceramic green body sample that is sandwiched between two conductive graphite dies that generate the electric field. Commercial versions of such systems exist, but they do not handle large-area coatings or complex shapes, and typically require a vacuum atmosphere. Published information shows such electric field-induced sintering has been applied to ceramic parts but not to coatings of ceramic on metals or other conductive substrates.

In a variation on SPS, several publications have demonstrated that so-called “flash sintering” can be used to consolidate ceramics at moderately low temperatures without the need for external pressure or a vacuum. Flash sintering uses an external heating source to bring the ambient temperature of the ceramic to a baseline temperature (for example, as low as ˜850-1000° C. for YSZ), and an electrical current flowing through the sample then consolidates the powder in a matter of seconds. Reduced sintering temperatures and times present a major opportunity for cost savings in materials processing. The actual temperature at which sintering occurs and the speed of sintering were shown to be controlled by the electric field strength. In each of the field-assisted processes above, the physical restriction of having two conductive electrodes limits the geometries of the ceramic parts being sintered. These existing bulk electric field methods also do not mention adding conductivity enhancers as does the current innovation, but rely on preheating the inorganic material high enough so that enough current can be properly sent through the material to be sintered.

Although common applications of ceramic coating may be satisfied by the various ceramic coating processes described, there is a continuing need for a method of low temperature sintering of ceramic material. Desirable capabilities include that it works well for large or contoured parts, and that can be applied under atmospheric conditions, free of the burdens of traditional vacuum chambers, can be processed at low temperatures and on substrates which cannot handle high temperatures, and which allows for the localized control of sintering or microstructures.

SUMMARY OF THE INVENTION

Speed of processing and using lower temperatures are both important factors for the processing of ceramic and other inorganic materials. Being able to control microstructure and porosity are additional benefits to many applications. The present invention enables such dramatic changes and capabilities by using flash sintering or electric current enhanced sintering-type methods to sinter materials that have little or no conductivity and adding small amounts of electrically conductive material to the green body material so that initial processing can start at lower temperatures. This conductivity additive sintering innovation covers an initial starting material composition, processing method, and final material form.

The present invention comprises a composition of material and method capable of being used to make sintered inorganic bodies or coatings onto conductive substrates. The technique and apparatus for creating the sintered body or coating from powder includes the use of a conductivity dopant to the initial powder mixture. The present invention is for enhancing the electric current sintering of electrically insulating or poorly conductive materials. Most non-metallic materials are electrically insulating. The material added can either burn out in the process or become part of the sintered material. Examples of material that can burn out (in part or fully) are forms of carbon and electrically conductive polymers. Examples of conductivity-enhancing materials that would be included in the coating, but not burn out, would include metals and electrically conductive (or semi-conductive) inorganics. How these are used will be covered in greater detail, but the key is that they are present in volumes enough to be much more conductive than the primary material to be sintered but just enough to be electrically resistive so when a current passes through them, they produce heat. This heat then creates the environment for the otherwise non-conductive material that is the prime objective of the sintering, to start passing current. The current as happens with the “flash sintering” processes discussed above which can require over 800 C initial heating prior to electric current sintering, and can with the present invention now be achieved starting at much lower temperatures and even ambient conditions.

In a more specific case of a flame with an electric plasma to sinter the powder onto a substrate surface, the substrate is electrically conductive or semi-conductive and is used as one electrode while the flame is used as the other electrode that is moved over the areas of the powder coating to be sintered. An electrical voltage is used to generate an electric plasma within the flame and a current through the coating, resulting in a powder-powder sintering and powder-substrate bonding. This sintering method is referred to as “flame-assisted flash sintering” (FAFS). Because the flame's trajectory and motion can be controlled via external motors, controllers, and the like, the area that is effectively sintered or morphologically changed can be very well controlled, with the sintered material areas can be just a micron or so away from non-consolidated materials.

Another embodiment of the process modulates the electrical properties of the starting powder coating to make it more electrically conductive so that sintering occurs more readily by the arc plasma. Yet another embodiment to make additive variations so that the FAFS process can be start without externally heating the substrate or coating or where the arc contacted areas are more intensely or fully sintered.

Another key aspect of the current innovation is the self-limiting nature of the conductivity additive. This can help ensure the electric current flows through more of the material to be sintered and is not limited to a concentrated path of lower conductivity. As the conductivity additive is either burned out or converted to lower conductivity the electric current path will move to other areas with lower resistance until that additive material is also transformed. In this way a more homogenous sintering occurs than without the additive. Thus one might still process at the same starting conditions as before, but end with a more desired end product. The novel aspect of the additive being self-limiting differentiates these electrical conductivity additives and this technique from traditional sintering aids, which are present simply to reduce the furnace temperature or control the phase.

If one wants particular areas to be more sintered than others, the amount or type of conductive additive can be varied. If one wants a more sintered zone or pattern, then the green body material would have the appropriate amounts of additives in those zones to enhance the electric current sintering to form the desired sintering variations. There are many existing methods to form variations in powder compositions and this innovation can make use of any of these. 3D printing can even form very complex shapes. The less sintered material could be moderately sintered and remain or very poorly sintered, in which case it be removed to leave just the more sintered material. This invention covers all dimensions and shapes of inorganic material to be sintered using an electric current to help in the sintering process.

One aspect of the current innovation is using a conductive additive that can burn out during the sintering process. The material is thus removed during processing much as binders, emulsifiers dispersants and other sintering mediums that also burn out during traditional thermal sintering.

Similar processing consideration must be made for these electrical sintering additives as have been applied to other burnout materials. Flash sintering is a fast process so more rapid diffusion or removal occurs, which most likely requires higher permeability. Carbon and range of conductive polymers can be used for this, with cost, functionality, and green body stability being additional factors in choosing the conductivity enhancing additive that can burn out. Because the material can be removed during sintering, the composition is less of a concern. We have found through experimentation that less than 3 wt % carbon black is best if fewer pores are desired in the sintered material. When FAFS processing above 5 wt % carbon with alumina, the coating can be removed partially, on a submillimeter scale, resulting in a more open microstructure, possibly by burn out gases, such as carbon dioxide causing spalling of some of the coating. If a more open structure is desired, then more than 5% is recommended. Even 1 wt % or less can be effective as a conductivity aid and less than ½% can be beneficial, compared with not using any. The conductivity additive does not have to form a continuous path because the current will arc between the grains of lower-resistance material. The key is a net reduction in the resistance so that the sintering is enhanced by reducing the initial sintering temperature, achieving a higher degree of sintering, achieving a more uniform sintering, and/or control of the microstructure.

Another aspect of the current innovation is using a conductivity-enhancing additive that does not burn out during the sintering process. The material thus stays during processing but may convert in form or properties, much as theramite or a compositional sintering aids during traditional thermal sintering. Similar processing considerations must be made for these electrical sintering additives as have been applied to other sintering aid materials. Flash sintering is a fast process, so more rapid diffusion or conversion occurs. Metals and a range of exothermic materials that conduct electricity can be used for this, with cost, effect on end composition, and green body stability being additional factors in choosing the conductivity-enhancing additive that does not burn out. For example, if high-purity sintered zirconium oxide (zirconia) is desired, then zirconium metal may be used as a conductivity enhancer, which would then be oxidized during sintering, leaving just sintered zirconia. If zirconia with some alumina is desired in the end, then zirconia with some aluminum metal conductivity aid could be used in the green state. Silica is a common sintering aid, so silicon could be used as an electrical conductivity aid for insulating ceramics materials, which could then convert to silica and form a low melting phase with the ceramic powder. Because the material is not removed during sintering the composition, its end effects must also be considered.

There can be advantages to forming a conductive surface for flash sintering that can be removed or converted as the process is run. In FAFS processing, such a conductive additive top layer can also limit spotting or pinning the plasma arc to a specific area of the coating, due to the conductive additive in the top layer being removed or converted and the electric arc moving to other areas of lower resistance where the additive is still present. One way of forming a conductive top layer during FAFS processing is to run the flame fuel-rich enough that some carbon is deposited, which is one preferred embodiment of the method and process. Higher currents or lower voltages can be achieved with such a top conductive layer. Better processing at lower temperatures is enabled. Rather than running the flame to create a more conductive top surface, a layer of conductive material can be put down by a range of methods on top of the inorganic material to be sintered. The advantage of having a burn off conductivity-enhancing material only on the surface is that the degassing is open to the air and does not cause spalling of the inorganic.

The FAFS process can sinter many materials on to a substrate surface through which milliamps of current can flow. Substrates can range from semiconductors, to carbon-based materials, to metals and slightly conductive ceramics. These can be pure materials, composites, or even just a layer on another material that is conductive enough to allow for milliamp currents to flow when large potentials (e.g., 100 to 5000 V) are applied. The substrate can be any shape or texture, but smoother surfaces and more uniform coatings provide for a more uniform sintering effect under consistent processing conditions. Self-limiting electrical additives of the present innovation can help to overcome many of these difficult situations by causing the current to pass through areas where the additives still remain, versus being pinned to a thin coating area or defect.

Powders to be sintered, with conductivity enhancers, may include semiconductors, ceramics, and composites. Examples of semiconductors include those listed in various semiconductor databases and numerous publications, and include pure materials and mixed-valence materials. Suitable ceramics include metal oxides or metalloid oxides and most compounds in publications or ceramic phase diagram databases. Composite examples include a combination of any of the above semiconductors, and/or ceramics, mixed with a conductivity enhancer like metals, such as stainless steel mixed with YSZ or alumina to better match thermal expansion coefficients or improve the bond strength to that of the substrate. The conductivity enhancer might not be totally consumed during flash sintering but may just be reacted on the surface of the powder grain. It can be beneficial that the process is self-limiting, such that the local conductivity is reduced after sufficient current has caused a desired amount of sintering to occur. To accomplish this, just a surface reaction layer of <100 nm in thickness can cause orders-of-magnitude increases in resistance, and thicker reaction layers of 250 nm, 500 nm, or above 1 micron can reduce conductivity still further.

It is not necessary that the reaction is just oxidation. It can involve a transition to a carbide, boride, nitride, or other compound. The consumption of interstitial material to form the desired compound is of benefit in that it can reduce residual pores resulting from residual gases. If the powder is prepared for flash sintering with the reactive gas as the primary interstitial fluid, then the process can result in higher densification and less outgassing. Examples would include pure oxygen with a metal to form an oxide, or pure nitrogen gas to form nitrides. A mixture of carbon and an element can react to form a carbide, but care must be taken to not have oxygen available or the carbon will most likely react to form a gas with the oxygen. Many of these oxygen-free mixtures are easier to achieve in thicker bodies processed in flash sintering sealed systems and may be difficult with FAFS when run in the open atmosphere. With FAFS, the base of the layer next to the substrate can have more of a non-oxide compound while the surface has more oxide.

Coatings and green bodies to be sintered may be composed of powders, binders, and coating-stabilizing additives, or can simply be inorganics of the final desired coating composition. The binder may be an organic material, such as a polymer, that is volatilized before or during the flash-sintering process. Alternatively, the binder may be an inorganic material, such as alumina phosphate, or a metal organic that can be integrated into the inorganic structure, in part or whole, during the sintering process.

Substrates may include metals, semiconductors, composites, conductor-coated insulators, and ceramics, so long as they conduct electricity better than the powder coating layer at sintering temperatures. Examples of suitable substrates include the semiconductors and metals above, with common ones including various grades of steel, titanium, aluminum, silver, precious metals, magnesium, silicon, carbonaceous materials, nitrides, carbides, borides, conductive oxides, superalloys, and composites containing these. The conductivity-enhancing additive would be more electrically conductive than the powder to be sintered. Thus, for an insulating powder, the conductive additive could even be a semiconductor, but to sinter metal powders, the conductivity additive would need to be more conductive, such as, for example, graphene, carbon nanotubes, or aluminum metal. The conductivity-enhancing additive material should be at least twice as conductive as the primary material, more preferably four times more conductive, and in many cases at least ten times more conductive. When the primary material has very low conductivity or is insulating, the conductivity aid will need to be at least 100 times more conductive, more preferably 1,000 times more conductive, and can easily be many more orders of magnitude more conductive. Suitable examples of metals include base metals and alloys, such as those listed in the ASTM database and other publications.

The initial coatings or green bodies may be deposited or formed onto the substrate by various methods, including Meyer Rod drawing, doctor-blade coating, dip-coating, spin-coating, aerosol-jet printing, inkjet printing, casting, pressing, and other processes.

The present invention introduces a composition and method to enhance flash sintering. The electrodes on either side of the material to be sintered can vary greatly as is published in a range of electric current or flashing sintering publications and papers, and can even be a flame or other conductive plasma on one or both sides of the material to be sintered. Other advantages of the present invention when the conductivity enhanced sintering is realized through sintering coating using the FAFS process are that it enables a lower cost and non-contact method of electric field sintering powder coatings, decreases sintering times, enables applications not suitable for vacuum chambers, is amenable to large and complex shapes, and can control the degree of sintering and grain growth over small scales through judicious selection of process parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows microstructures of FAFS-process alumina coatings on 430 grade stainless steel substrates using varying degrees of conductive carbon additive. The amounts of carbon used in each image are: FIG. 1 a: 1.0 wt %, FIG. 1 b: 3 wt %, and FIG. 1 c: 10 wt %. The weight % calculation refers to the weight of the additive with respect to all solid material.

FIG. 1a shows optical micrographs from a low power microscope as well as one photograph of the entire processed 3″×1″ sample with 1 wt % carbon additive. The scale bars on the optical micrographs all indicate 500 μm. The darker regions are where the FAFS arc processed the coating.

FIG. 1b shows an optical micrograph (top left) and SEM images of a FAFS-processed sample with 3 wt % carbon. The SEM images show regions of dense alumina but also reveal porosity possibly from the burnout of the higher amount of carbon additive.

FIG. 1c shows optical microscope images of a FAFS-processed sample with 10 wt % carbon additive. The surface is rougher possibly due to the increased amount of additive that is burned out during the FAFS processing. The scale bars all indicate 500 μm.

FIG. 2 shows a conductive additive FAFS-processed alumina/YSZ composite coating on 430 grade stainless steel.

FIG. 3 shows a conductive additive FAFS-processed alumina coating on an aluminum substrate.

FIG. 4a shows a conductive additive FAFS processed sample where the flame with steady current was traversed in an offset raster pattern. This is before any attempt to remove loose material.

FIG. 4b shows the same area as taken from FIG. 4a , but after washing the surface in running water with a plastic bristle brush. One can see the full removed areas and the unaffected FAFS-processed areas showing significant sintering of the material (about 1 s or less sintering time) as the FAFS flame and arc was traversed in the pattern seen.

FIG. 4c shows the same area as taken from FIG. 1b but at a higher magnification to see better that there is a near-vertical step up of the coating in this example.

FIGS. 5a-c show microscope images of a freestanding thick film of FAFS-processed alumina sintered powder, which was separated from the base aluminum metal substrate. All scale bars indicate 1 mm (1000 microns). Processed similar to FIG. 3 but thicker layer and shows that conductive additive electric arc processed material can be independent and not attached to the initial electrode. In this case is in the form of a freestanding film but the invention also covers a 3 dimensional form or body.

FIGS. 6a-b are optical microscope images from Example 5 showing the green material in FIG. 6a prior to electric current sintering and FIG. 6b at the same magnification (20 μm scale bar). After FAFS processing, the reflective aluminum metal additive was converted to a less reflective oxide. Note that a lack of pore structure was present in many carbon additive examples. The aluminum transfers gas into a solid whereas carbon can create gaseous carbon oxide products.

DETAILED DESCRIPTION OF THE INVENTION

The images shown in FIGS. 1 to 5 illustrate achievable electric current sintering without any heater or furnace. These figures are explained with the captions and the description of examples. While all of these examples are using the FAFS process on coatings, bodies or 3D shapes can be processed also. While all these examples of plasma arc sintering are of coatings it could be applied to thicker materials and one could even process materials with both electrodes being plasmas.

FIG. 1-5 are self explanatory along with the figure captions and are more fully explained in the examples.

Although electric current forms of sintering are capable of being used in a vacuum environment, with flames being stable to at least 15 torr for FAFS and other plasmas stable to much lower pressures, it is practically preferred for use in non-vacuum environments, because this reduces equipment and costs, enabling in-place applications, such as very large components, repair applications, and applications requiring challenging orientations, such as vertical or overhead surface coatings. Vacuum processing does have advantages. When processing samples in a vacuum there will reduced amounts of gases inhibiting densification, so that the quality and density of the resulting sintered material can be higher and oxidation can be more readily controlled.

The plasma electrode for sintering of material can also occur with a plasma not formed by a flame. There are many traditional forms of plasma and some that produce local control of the plasma. TIG torches produce an electric arc at atmospheric pressure, and there are more uniform field plasmas that are formed at reduced pressures. An ionizable gas is generally used to produce these plasmas. While the preferred method of making the electric current for sintering is a flame, these other forms of plasmas that yield an electric current flow through gas can also be used. To help control the location of the current flow yielding the sintering, the surrounding gas can be less ionizable than the surrounding gas. The surrounding gas should be preferably at least half as ionizable, and even more preferred at least 80% less ionizable. The voltage source should be close to or in the more ionizable gas flow, just as it should be for the flame when it is used to complete the electric circuit. Then motion of the ionizable gas is relative to the substrate, and can be moved as desired to yield the areas of sintering wanted similar to when using the flame. A flame is an ionizable gas and is a form of chemically ionized gas which makes the conduction of electricity easy. This makes the flame readily electrically ionizable. Ionizing gasses, without a flame, may times require an initial high voltage to initiate the plasma. The electrically conductive additive can also be used with conventional solid electrodes.

Additionally, although electric current sintering with conductivity enhancers was demonstrated for coated metals, it is applicable to any substrate having electrically even the smallest conductive properties. One only needs to pass milliamps of current through the substrate or a coating on the substrate with high potential being applied.

Flame-assisted flash sintering may also be used for bonding or welding of material(s) to electron passing surfaces. In this case, the material could be in a green, partially sintered, or fully sintered state. During bonding of the material, the material may also undergo partial or full sintering or grain growth. The material to be welded may be in the form of a green-state coating, as a tape or sheet, or a solid, shaped and attached to the electrode surface, or a secondary plasma electrode could be used to complete the circuit for current to flow. The current can be DC or AC in form.

It is possible to sinter only desired areas with the FAFS process. If the material is in coating form, specific areas of the coating may be welded and sintered to the substrate by FAFS, and the unwelded and unsintered ceramic can then be removed to expose the substrate in areas where no coating is desired. Unsintered material can be removed by many different processes, including washing, scrubbing, blowing, vibration, or other known cleaning or removal methods. The FAFS process can be localized and it may be easier to define shapes and areas for the coating to remain than to mask or otherwise limit where the material is to be applied to the substrate. A more complex pattern can also be made by altering the conductivity enhancer content and combining this with selective exposure to the plasma electrode.

It is also possible to run the current sintering process such that the surface is sintered but the bonding to the substrate is weak so that a sintered free-standing sheet is created as shown in FIG. 5. When subjected to force, such as thermal expansion strain, the sintered layer can delaminate forming very thin sheets of ceramic. Thus the selective sintering can be both horizontally and/or vertically controlled. Another example of desired vertical control would be a denser surface to provide protection but a more porous (less sintered but still adherent) under layer to have other properties such as strain tolerance or thermal insulation.

For the examples described, the following preparations were made. A conductivity-enhancing slurry was made for processing. The slurry or paste can be made in many ways. The following is simply the method used and does not limit the breadth of processing.

Oxide powder and conductive powder was added to a solvent and dispersed with an ultrasonic probe (e.g., Hielscher UIP100hd). Slurries were sonicated for ˜10 min at ˜75% amplitude while manually stirred in an ice bath to minimize solvent evaporation. Slurries were cooled to room temperature via the ice bath prior to use. Slurries have also been made my rolling with grinding media and rapid rotation mixing methods, but most any mixing technique that makes a stable slurry, dispersion or ink can be used. The end fractional amounts are approximate because some solvent is lost.

Example slurry recipes:

Alumina

35-40 g n-butanol solvent

25 g of Baikalox BMA-15 alumina powder

0.25-2.75 g Timcal SuperC65 Carbon Black powder (corresponding to 1-10 wt % of solids or 2-20 vol % of solids).

Alumina/YSZ Composite

28.3 g n-butanol solvent

20.5 g Tosoh TZ3YS20A YSZ/alumina powder

0.3 g Timcal SuperC65 Carbon Black powder

0.5 g polyvinylpyrrolidone binder

The metal substrate was prepared as follows. After cutting to size and removal of masking adhesive, 0.075″ or 0.125″ thick substrates were cleaned with distilled ethanol in an ultrasonic bath cleaner for ˜15 min to remove any residual adhesive remaining on the substrate surface. After cleaning, substrates were rinsed in reverse osmosis or distilled water and sprayed dry with compressed air. Care was taken to keep dust particles and drying marks to a minimum.

The slurry was applied as a coating onto the metal substrate as follows. Clean substrates were placed onto flattened sheets of aluminum foil and then onto the glass coating plate of a bench top automated coating system. A wound-wire Meyer rod was cleaned by bath sonication in distilled ethanol and sprayed dry with compressed air. Cleaning cycles with ethanol were continued until the rod was completely clear of debris. With both substrate and coating rod cleaned, the rod was inserted into the holder and lowered onto the substrate. Slurry was pipetted onto the substrate and the coating rod was drawn across. After coating, wet samples were transferred to a hot plate and dried for ˜5 min at ˜80-130° C. Once dry, coated substrates were inspected manually for defects and any excess coating was removed from the substrate back with a dust-free wipe.

Typical coating thicknesses for examples of alumina and YSZ/alumina composite samples were ˜12-15 μm. A wide range of thicknesses have been processed. For the listed examples, the following pieces of equipment were used when needed, but these items could be replaced with other equipment or set of components that perform similar functions:

-   -   1. The flame equipment used was a custom-built torch assembly         consisting of a central flame (such as that produced by a Smith         Little Torch with #5 tip) surrounded by a more diffuse annular         flame. The latter flame is referred to as an “auxiliary flame”         source because its primary purpose is to broaden the heat         distribution and not to sinter the coating or deliver the plasma         arc. The central flame torch typically protrudes from the         auxiliary flame burner by ˜2 mm.     -   2. The voltage or current supplies used were a Stanford PS300         high voltage power supply, an Acopian P01HP60 high voltage power         supply, and a Hoefer PS2500 high voltage power supply; they were         used interchangeably and others can be used.     -   3. Alicat mass flow controllers, 0.5 SLPM and 2.0 SLPM (propane         as the fuel gas and as the oxidizer O₂, respectively), as well         as manual rotameters.     -   4. Standard (industrial) grade propane, methane, air and oxygen         gases.     -   5. The substrate was placed onto a sheet of silver foil that was         connected to the power source.

Using the above equipment and prepared materials, the examples listed below were made with the following process. Single-sided coated substrates were placed onto the silver sheet without clamping. The silver sheet was connected to electrical ground through a 1.5 to 100 kΩ ballast resistor, and this was positioned atop either a larger metal plate or directly on a ceramic sheet to electrically isolate the part and electrode from the rest of the system. Electrical grounding issues may occur if the processing circuit electrically interacts with motion or other equipment. The ballast resistor is connected in series with the negative side of the power supply and serves to restrict the maximum current in the circuit. The ballast resistor is intentionally placed on the negative side of the circuit so that the positive voltage applied to the torch is not attenuated through additional resistance before any plasma is ignited. Note that the ballast resistor must be of a sufficient wattage rating to handle the power delivered to it: a 25-W ballast resistor of 1.5-150 kΩ resistance was used. The resistor was found to help stabilize the power flow, but other means to finely control the electrical power, such as different circuitry or power supplies, can replace this or alter its value. Other resistor values can be used or other circuits could be used to eliminate the need for the ballast resistor. No substrate heater was used in any of the examples to show the extent of processing capabilities, but heaters could be used.

The torch is clamped by an electrically insulating fixture onto a two-axis linear motion stage above, in the vicinity of the substrate holder and coated substrate. It is important that the torch be clamped using electrically insulating materials to prevent high voltage from being transferred to the motion system and thus the rest of the assembly. This is important both for operator safety and practical purposes, to avoid shorting the power supply to ground. The high voltage is supplied to the torch by means of an electrical spade lug that is silver-soldered to the body of the electrically conductive torch tip. A matching spade connector crimped onto the end of a cable (capable of withstanding high voltages) mates to the lug; this cable is connected to the positive terminal of the power supply.

A motion trajectory for the torch is determined and programmed into software that controls the motion of the entire three-axis system. It is useful to define a three-axis Cartesian coordinate system consisting of x, y, and z axes, such that the z-axis is parallel to the common understanding of vertical (up and down) movement, and the x-y plane is orthogonal to the z-axis. The trajectory used in all experiments to date consisted of holding the torch at a fixed height (z position) above the substrate surface while rastering along at a fixed speed in the x-y plane. At the end of each raster line (assuming rastering along the major axis, x), the substrate position is indexed in y and the torch returns to the initial x position. This pattern is repeated a number of times until the desired number of scan lines have been executed. Practical values used in our example experiments are shown in the table below, but wider ranges also function.

z height 2.0-5.0 mm z trajectory speed 100-200 mm/min x trajectory speed 20-200 mm/min y trajectory speed 20-200 mm/min x scan length 25-75 mm y index position length 0.5-2.0 mm

Before electrically energizing the circuit, combustible gases are delivered to the torch and the flame is lit. Successful methods of gas delivery in these experiments included manual rotameter flow devices as well as electronic mass flow controllers designed to deliver precise amounts of gas. The latter has the advantage of creating a very stable flame, which is preferred to support a stable plasma. Fuel and oxidizing gases were delivered through separate mass flow controllers or rotameters and premixed within the torch assembly. Propane and oxygen were used as the primary fuel gases in these experiments. Methane was also tested as an acceptable fuel gas but not in any of the incorporated examples. Air, oxygen and argon mixed with oxygen, were demonstrated to be functional with the FAFS process. Various gases (or other fuel gases, such as butane and hydrogen) may be used once appropriate experimental conditions are ascertained.

By setting a voltage on the power supply, the FAFS circuit was energized. All experiments to date were performed as described above with the torch at a positive electrical potential with respect to the substrate base electrode, and, by extension, the substrate. It may be that reversing the polarity of this voltage may show comparable or even greater success than the present configuration. Changing the placement of the ballast resistor to the positive side of the circuit is also a modification that may be contemplated with the experimental parameters. It is noted that the torch is only electrically energized after lighting the combustible gases for safety reasons.

Voltages between 500 and 2000 V were applied to the torch (with respect to the substrate) to achieve currents ranging from 1 to 150 mA. The power supply may be controlled in constant current or constant voltage mode, as outlined in the proceeding examples. In theory, constant current mode should be preferable because the temperature increase due to the electrical current within the ceramic is proportional to power, and power is proportional to the square of the current multiplied by the ceramic resistance. The coating resistance varies as the conductivity enhancers is changed and then as the temperature of the ceramic increases. A change in current has a significant effect on the deposited power, and thus the temperature increase, within the ceramic. Variable sintering can be achieved by purposefully adjusting current or voltage while processing, in which case non-constant electric potentials or currents are not just desired but purposefully created.

Once a flame is lit and the torch is electrically energized, the scanning motion trajectory begins, with the torch descending in the z-axis until it reaches the fixed height at which it will begin the x-y scanning motion. As the torch descends, it is sometimes necessary to also execute some x-y scanning motion so that a single point on the substrate does not get too hot. A typical value for this height is 2.5 mm, which provides enough space for stable combustion of the fuel-gas mixture before the primary combustion zone contacts the substrate surface. The z-height is an important parameter in the FAFS process, because the hottest section of the flame can reach temperatures in excess of 2,000° C., under certain combustion conditions, sufficient to oxidize, damage, or melt the metal substrate. For this specific flame, use at a height of <1 mm may damage the coating due to erosion or extreme heat stress, while a height of greater than 5 mm may be too far away from the surface to generate a stable plasma arc using the current torch apparatus. Other flames and torches will require different surface offsets which can be determined by experimentation.

The nature of the arc differs substantially between no conductivity aid and with the amount of the conductivity enhancers used. We were not able to achieve a stable arc or one that would move uniformly over the coating without a substrate heater set point above 600° C. For YSZ, conditions with low temperatures tended to cause coating spalling or delamination. The plasma arc, which extended visibly from the torch tip to the substrate, moved rapidly and sporadically within the lateral extent of the combustion zone. For a x-y scanning speed of 25.4 mm/min, the 0.1-0.2 mm diameter plasma arc moved in such a way as to expose 50-80% of the ceramic coating within the lateral extent of the combustion zone. The arc did not show any noticeable sparking but just moved rapidly.

Alumina with some carbon added, on the other hand, could be processed when the substrate is not heated with the substrate heater and the sample was at ambient temperature prior to processing. Using a current set point of 15 mA in constant current mode, the voltage obtained was of the order of 2,000 V using a 100k ohm ballast resistor. The nature of the plasma arc was fundamentally different than that of the 8YSZ case; luminescence was much less and a “shower” of multiple current arcs appeared rather than a single one. A high-frequency audible “hissing” sound was also typically heard in this case. As the level of carbon black was increased sparking of minute embers could be noticed emitting from the surface and sparking increased with increasing carbon content.

Once the scanning trajectory was complete, samples were either allowed to cool slowly to room temperature while residing on the substrate plate, or were instantly removed to a cooler surface for examination. There was no noticeable difference observed between the two different cooling rates, although one may be preferable to the other upon closer examination in the future.

EXAMPLE 1

TABLE 1 Experimental parameters for Example 1. Flame + Flame + ceramic Heater Traverse ceramic electrical SP Voltage Current speed resistance power (° C.) (V) (mA) (mm/min) (kΩ) (W) N/A 2030 11.0 25 85 10.3 Auxiliary Nozzle nozzle Propane O₂ Powder Flame + height H height flow flow size plasma arc (mm) (mm) (sccm) (sccm) (nm) diameter 4.5 5.5 160 440 150 0.5-1.0

FIG. 1a shows an actual sample processed with no heating other than the process using the conditions in Table 1. The carbon additive amount in this example was 1 wt % solids. FIGS. 1b and 1c show coatings with higher additive concentrations of 3 wt % and 5 wt %, respectively, and were processed with conditions similar to those listed in Table 1. From the figures it is evident that increasing the amount of carbon content gives rise to a rougher surface with increased porosity. Numerous operating conditions were tried with the exact same FAFS set-up and coating material except no carbon added, which resulted in wide current and voltage changes, significant amount of spotting or pinning, the arc extending sometimes even a few millimeters from the inner flame, and very poor or no sintering over much of the area.

EXAMPLE 2

TABLE 2 Experimental Parameters for Example 2 Flame + Flame + ceramic Heater Traverse ceramic electrical SP Voltage Current speed resistance power (° C.) (V) (mA) (mm/min) (kΩ) (W) N/A 1600 10.0 50 60 6.0 Auxiliary Flame + Nozzle nozzle Propane O₂ Powder plasma arc height H height flow flow size diameter (mm) (mm) (sccm) (sccm) (nm) (mm) 2.0 15.5 170 210 180 0.5-1.0

FIG. 2 shows an actual sample processed with the experimental parameters in Table 2. The coating material in this case is an alumina/YSZ composite as described in the example slurry recipes above. The pre-processed coating is black from the carbon additive but turns white after processing as the conductive additive oxidizes and leaves the coating. The SEM image in FIG. 2 shows the sintering that had taken place between particles.

EXAMPLE 3

TABLE 3 Experimental Parameters for Example 3 Flame + Flame + ceramic Heater Traverse ceramic electrical SP Voltage Current speed resistance power (° C.) (V) (mA) (mm/min) (kΩ) (W) N/A 2030 13.5 50 50 9.2 Auxiliary Flame + Nozzle nozzle Propane O₂ Powder plasma arc height H height flow flow size diameter (mm) (mm) (sccm) (sccm) (nm) (mm) 2.3 9.3 70 120 150 0.5-1.0

FIG. 3 shows an actual sample processed with the parameters given in Table 3. The coating material is Alumina made with the example slurry recipe described above (3 wt % solids carbon additive), and the substrate is aluminum. This example shows that sintering is occurring at very low temperature conditions because the substrates will melt below 600° C. Higher magnification reveals morphologies indicative of liquid phase alumina being present during the sintering. Alumina melts at ˜2070° C., so this suggests that very rapid sintering with additives can achieve extreme localized conditions.

EXAMPLE 4

TABLE 4 Experimental parameters for Example 4. Flame + Flame + ceramic Heater Traverse ceramic electrical SP Voltage Current speed resistance power (° C.) (V) (mA) (mm/min) (kΩ) (W) N/A 1500 8.0 50 50-100 3.2-6.4 Auxiliary Flame + Nozzle nozzle Propane O₂ Powder plasma arc height H height flow flow size diameter (mm) (mm) (sccm) (sccm) (nm) (mm) 4.5 5.5 175 390 150 0.8-1.0

FIG. 4 shows images of actual test results from the experimental parameters and conditions shown in Table 4. The coating was alumina with carbon added. In this case the electric plasma generally followed near the back edge of the intersection of the inner flame contact with coating surface. When the width of the inner flame contact area was increased (by changing the flame or the flame height) the width of the electric arc processing and the resulting sintering also could be changed.

FIG. 4a shows the sample as-processed. The processing lines are visible. FIG. 4b shows the same sample after rinsing with water and brushing loose powder from the surface with a plastic brush. The processed lines resist the brush and remain adhered to the surface, a testament to their bonding to the metal substrate. FIG. 4c shows a higher magnification view of the same sample.

EXAMPLE 5

FIG. 6b shows an actual sample processed using the conditions in Table 5. The aluminum powder conductivity-enhancing additive amount in this example was 15 wt % solid. The aluminum powder additive was included into the slurry formulation to improve the electrical conductivity of the green coatings, to improve arc stability. Furthermore, the aluminum powder additive is self-limiting after processing since it oxidizes from Al to AlO₃. As shown in FIG. 6, the green coatings had shiny spots of reflective aluminum while the processed FAFS portions showed a more uniform layer of all ceramic materials. Numerous operating conditions were tried with the same FAFS set-up and coating material except that no aluminum was added, which resulted in wide current and voltage changes, significant amounts of spotting or pinning, the arc extending sometimes even a few millimeters from the inner flame, and very poor or no sintering over much of the area. A process change was made to add a plate beneath the sample to moderate its temperature. This plate had water flowing through it at ˜50° C. and thermally and electrically conductive paste was used to ensure good heat transfer with the sample.

TABLE 5 Experimental parameters for Example 5 Flame + Flame + ceramic Cooling Traverse ceramic electrical plate Voltage Current speed resistance power (° C.) (V) (mA) (mm/min) (kΩ) (W) 50 ~1000 100 100 50-100 ~100 Auxiliary Flame + Nozzle nozzle Propane O₂ Powder plasma arc height H height flow flow size diameter (mm) (mm) (sccm) (sccm) (nm) (mm) 3 5.5 .2 .34 800 0.8-1.0

The coating was Tosoh TZ 3YS20A with aluminum added. In this case the electric plasma generally followed near the middle intersection of the inner flame contact with coating surface. When the width of the inner flame contact area was increased (by changing the flame or the flame height) the width of the electric arc processing and the resulting sintering also could be changed.

EXAMPLE 6

Yet another sample processed with metal additive using the conditions in Table (6). The aluminum powder additive amount in this example was 15 wt % solids along with a 2 wt % CTAB addition. The CTAB dispersant as included into the slurry formulation to improve the stability of the formulated slurries. Furthermore, the CTAB dispersant burnout during the processing, and was not detrimental to the sintered coating.

TABLE 6 Experimental parameters for Example 6. Flame + Flame + ceramic Cooling Traverse ceramic electrical plate Voltage Current speed resistance power (° C.) (V) (mA) (mm/min) (kΩ) (W) 50 ~1000 150 100 50-100 ~150 Auxiliary Flame + Nozzle nozzle Methane O₂ Powder plasma arc height H height flow flow size diameter (mm) (mm) (sccm) (sccm) (nm) (mm) 1.5 3 .2 .6 800 0.8-1.0

The coating was thinner and had a lower viscosity with the CTAB additions. The slurry stability was improved with the addition of CTAB and would resist agglomeration.

EXAMPLE 7

The aluminum powder additive amount in this example was 15 wt % solid. This example is similar to Example 5 but was run at lower current conditions. Choosing the optimal current conditions is important. Both resulted in sintering but higher currents generally yield higher sintering. Currents that are to very high can cause blackening of the coating that may normally be white.

TABLE 7 Experimental parameters for Example 7. Flame + Flame + ceramic Cooling Traverse ceramic electrical plate Voltage Current speed resistance power (° C.) (V) (mA) (mm/min) (kΩ) (W) 50 ~1000 40 100 50-100 ~400 Auxiliary Flame + Nozzle nozzle Propane O₂ Powder plasma arc height H height flow flow size diameter (mm) (mm) (sccm) (sccm) (nm) (mm) 1.5 3 .1 .5 800 0.8-1.0

Processing the green coatings at lower currents can achieve sintering conditions, and reduce risk of pinning of the electric arc.

The results achieved differ widely from those achieved by flame plasma alone. On both YSZ-alumina and alumina coatings, flame-only processing was performed and nominal or no sintering was achieved and the adhesion was poor. With no carbon or other conductivity enhancing additions these coatings could not be processed unless the substrate as well heated prior to FAFS processing. The ceramics being processed either become ionically conductive at higher temperature or have reduced electrical breakdown. The conductivity additive helps to overcome this initial barrier to allowing current to pass. Once initiated the electric current sintering produces high local temperatures as well as charged atoms and free electrons that further help diffusion and sintering.

The FAFS process uses a flame to define a path where the plasma arc is restricted and then the flame can be traversed or moved relatively over the area to be treated. Additionally, the flame has some conductivity and can support a lower resistance path so that lower power plasma arc can exist versus non-flame-based plasma arcs. The plasma is a composite of both a flame plasma and an electric arc plasma, which enables a lower current flow than is required to sustain a pure electric arc so that the right amount of energy to properly sinter, without damaging the powder coating or substrate, can be achieved more readily. With proper equipment and setting a non-flame ‘pure’ arc plasma could achieve sintering when using the conductivity additives. The current and voltage required to form an arc plasma is known to vary with the composition of the gas medium. Another significant factor is pressure, and under reduced pressure electric plasmas are more stable at lower current flows. Of course, any air that might be entrained should be included in the gas mix, so some form of enclosure or localized gas flow control would be necessary. The flame or heater helps to bring the powder material up to a temperature where electric current sintering can be effective, but the electric conductivity enhancer minimizes or eliminates the need for preheating needed for good sintering.

Even though the conductivity additive helps minimize spotting at coating defects, the powder coating should be of reasonable quality without coating material lacking in the area of processing. While the flame does control the zone of the electric plasma are, if there are holes or cracks in the coating, the arc can try to move to these areas of a lower resistance path and will jump over or move quickly by areas where the coating has significantly higher resistance.

Powder mixture contaminants and poor mixing should be minimized, as is the case for most sintering methods. Some contaminants might dramatically alter the melting point or resistance of the mixture and result in different morphologies or properties as well as difficult to control currents or voltages. As with many processes, cleaner or more consistent properties are better. There could be benefits to some additional materials on processing, but uniformity is helpful in maintaining operating conditions.

While the examples are all based on FAFS processing, other electric current sintering processes for coatings or bodies will also be positively affected by the use of electrically conductive additives to the pre-processed medium. In many cases, it is simply a matter of applying higher voltage and current so that a larger volume of material with further electrode separations is sintered. There are numerous publications that cover the possessing conditions for inorganic bodies to be sintered. As stated earlier the general differences with using the conductivity additive to the initial mixture are lower initial temperatures, lower voltages and more uniform sintering. With thicker material the diffusion will be more difficult and if gas volumes increase then spalling will be more of an issue. Self-limiting reactions might be by the conversion of any free oxygen to carbon oxides with nominal volume change after which the carbon can remain or react to form carbides. There can be an actual decrease in pore gas by reacting with a solid in a more conductive elemental state to an oxide, nitride or other compound with the pore gas actually being absorbed into a solid compound. This will actually be the opposite effect of spalling and cause reduced porosity and densification. Prior to sintering it could be beneficial to vacuum infiltrate the pores with the desired reactive gas and have the proper porosity to react with the conductivity additive so little gas remains and high density is achieved. While more important when the material is thicker this pore reduction by balancing gas reactive species could also be very important for coatings also.

Embodiments of the present invention include at least the following:

-   -   1. A method of electric current-induced sintering of materials         with varying degrees of electrical conductivity, the method         comprising:         -   a) providing a first electrically conductive surface or             fluid acting as a first electrode,         -   b) providing a powder having a plurality of particles of             which there is a larger amount of more electrically             insulating material and a smaller amount of more             electrically conductive material,         -   c) attaching a green body to said powder or disposing said             powder on said first surface to form a powder layer on said             first electrode,         -   d) providing an opposing electrically conductive surface or             fluid opposing the first conductive surface, acting as a             second electrode, and         -   e) creating an electric circuit that connects the electrodes             so that a current passes through the material between the             electrodes, causing said materials to sinter.     -   2. The method of 1. (above) wherein said more electrically         conductive material becomes less electrically conductive during         the sintering.     -   3. The method of 1. (above) wherein one or both of the         electrodes are ionic gases.     -   4. The method of 3. (above) wherein said ionic gas is a flame in         the temperature range of 1000° C. to 3000° C. and produces         chemically and thermally generated ions as constituents of a         plasma.     -   5. The method of 4. (above) wherein said flame produces         chemically and thermally generated ions as constituents of a         flame plasma and the electrical potential creates an arc-like         plasma in the flame that is rastered over the coating or body of         material being sintered.     -   6. The method of 5. (above) wherein the gas flow over the         surface is moved such that the area of current flow does not         cover all the coating, resulting in areas of more sintered         material where the gas makes contact with the coating.     -   7. The method of 1. (above) wherein sintering occurs at a         voltage at least 20% less than of that possible without a         conductivity-enhancing additive in the composition.     -   8. The method of 1. (above) wherein the electric arc is         traversed over select areas where coating material is desired to         remain for the product being made and subsequently the more         sintered powder layer is removed when the substrate is subjected         to a cleaning or unsintered powder removal method.     -   9. The method of 1. (above) wherein the material being sintered         is composed of areas with different compositions.     -   10. A process for sintering a powder comprising         -   a) having two electrodes on opposing sides of a material to             be sintered,         -   b) providing between said electrodes a powder having a             plurality of particles of which there is a larger amount of             less electrically conductive material and a smaller amount             of more electrically conductive material, and         -   c) passing a current through said powder sufficient to             sinter said powder.     -   11. The process of 10. (above) wherein said more electrically         conductive material becomes less electrically conductive during         the sintering.     -   12. The process of 10. (above) wherein the sintered material has         an electrical conductivity at least 10 times less than that of         said material to be sintered.     -   13. The process of 10. (above) wherein said more electrically         conductive material is carbonaceous.     -   14. The process of 10. (above) wherein said more electrically         conductive material is a mixture of electrically conductive         materials.     -   15. The process of 10. (above) wherein said less electrically         conductive material is a mixture of less conductive materials.     -   16. The process of 10. (above) wherein said sintering is started         at below 600° C.     -   17. The process of 10. (above) additionally comprising an         electrical circuit configured to apply at least part of the         range of 100 V to 5000 V of electrical potential and control a         desired flow of current of 1 mA to 5 A through said electrodes.     -   18. A composition of starting materials for use in         electric-current sintering comprising:         -   a) a majority of a first material comprising an inorganic             powder of at least one composition,         -   b) a minority of a second material at least three times more             conductive than said first material,         -   c) so that said starting material can be formed into a body             or coated onto a surface when sintered.     -   19. The composition of 18. (above) wherein the said second         material is at least 10 times more conductive than said first         material.     -   20. The composition of 18. (above) wherein the second material         is a metalloid, metal, or semiconductor.

Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

All documents, books, manuals, papers, patents, published patent applications, guides, abstracts, and other references cited herein are incorporated by reference in their entirety. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims. 

1. A method of electric current-induced sintering of materials with varying degrees of electrical conductivity, the method comprising: a) providing a first electrically conductive surface or fluid acting as a first electrode, b) providing a powder having a plurality of particles of which there is a larger amount of more electrically insulating material and a smaller amount of more electrically conductive material, c) attaching a green body to said powder or disposing said powder on said first surface to form a powder layer on said first electrode, d) providing an opposing electrically conductive surface or fluid opposing the first conductive surface, acting as a second electrode, and e) creating an electric circuit that connects the electrodes so that a current passes through the material between the electrodes, causing said materials to sinter.
 2. The method of claim 1 wherein said more electrically conductive material becomes less electrically conductive during the sintering.
 3. The method of claim 1 wherein one or both of the electrodes are ionic gases.
 4. The method of claim 3 wherein said ionic gas is a flame in the temperature range of 1000° C. to 3000° C. and produces chemically and thermally generated ions as constituents of a plasma.
 5. The method of claim 4 wherein said flame produces chemically and thermally generated ions as constituents of a flame plasma and the electrical potential creates an arc-like plasma in the flame that is rastered over the coating or body of material being sintered.
 6. The method of claim 5 wherein the gas flow over the surface is moved such that the area of current flow does not cover all the coating, resulting in areas of more sintered material where the gas makes contact with the coating.
 7. The method of claim 1 wherein sintering occurs at a voltage at least 20% less than of that possible without a conductivity-enhancing additive in the composition.
 8. The method of claim 1 wherein the electric arc is traversed over select areas where coating material is desired to remain for the product being made and subsequently the more sintered powder layer is removed when the substrate is subjected to a cleaning or unsintered powder removal method.
 9. The method of claim 1 wherein the material being sintered is composed of areas with different compositions.
 10. A process for sintering a powder comprising a) having two electrodes on opposing sides of a material to be sintered, b) providing between said electrodes a powder having a plurality of particles of which there is a larger amount of less electrically conductive material and a smaller amount of more electrically conductive material, and c) passing a current through said powder sufficient to sinter said powder.
 11. The process of claim 10 wherein said more electrically conductive material becomes less electrically conductive during the sintering.
 12. The process of claim 10 wherein the sintered material has an electrical conductivity at least 10 times less than that of said material to be sintered.
 13. The process of claim 10 wherein said more electrically conductive material is carbonaceous.
 14. The process of claim 10 wherein said more electrically conductive material is a mixture of electrically conductive materials.
 15. The process of claim 10 wherein said less electrically conductive material is a mixture of less conductive materials.
 16. The process of claim 10 wherein said sintering is started at below 600° C.
 17. The process of claim 10 additionally comprising an electrical circuit configured to apply at least part of the range of 100 V to 5000 V of electrical potential and control a desired flow of current of 1 mA to 5 A through said electrodes.
 18. A composition of starting materials for use in electric-current sintering comprising: a) a majority of a first material comprising an inorganic powder of at least one composition, b) a minority of a second material at least three times more conductive than said first material, c) so that said starting material can be formed into a body or coated onto a surface when sintered.
 19. The composition of claim 18 wherein the said second material is at least 10 times more conductive than said first material.
 20. The composition of claim 18 wherein the second material is a metalloid, metal, or semiconductor. 